Effects of the improper modeling of viscous damping on the first-mode and higher-mode dominated responses of base-isolated buildings

In many finite element platforms, a classical global damping matrix based on the elastic stiffness of the system (including isolators) is usually developed as part of the solution to the equations of motion of base-isolated buildings. The conducted analytical and numerical investigations illustrate that this approach can lead to the introduction of unintended damping to the first and higher vibration modes and the spurious suppression of the respective structural responses. A similar shortcoming might be observed even when a nonclassical damping model (ie, an assembly of the superstructure and isolation system damping sub-matrices) is used. For example, the use of Rayleigh damping approach to develop the superstructure damping sub-matrix can lead to the undesired addition of damping to the isolated mode arising from the mass-proportional component of the superstructure damping. On the other hand, the improper use of nonclassical stiffness-proportional damping (eg, determining the proportional damping coefficient, β_k , based on the first mode) can result in assigning significant damping to the higher-modes and the unintended mitigation of the higher-mode responses. Results show that a nonclassical stiffness-proportional model in which β_k is determined based on the second modal period of a base-isolated building can reasonably specify the intended damping to the higher modes without imparting undesirable damping to the first mode. The nonclassical stiffness-proportional damping can be introduced to the numerical model through explicit viscous damper elements attached between adjacent floors. In structural analysis software such as SAP2000®, the desired nonclassical damping can be also modeled through specifying damping solely to the superstructure material.     

重复循环荷载作用下原状黄土动力特性试验研究

为探究重复循环荷载作用下含水率、固结压力对原状黄土动力特性、变形性状的影响规律,利用英国GDS双向动态三轴试验系统模拟交通荷载,对海东地区原状黄土进行动三轴试验研究。试验结果表明：单一重复循环动荷载作用下,在加载初期,海东地区原状黄土的轴向动应变随着循环次数的增大急剧增加,后期缓慢增加至趋于稳定,即发生应变硬化现象。在加载初期,含水率、固结压力对原状黄土的轴向动应变无明显影响,当循环次数N>400时,原状黄土的轴向动应变随含水率的升高而增加,随固结压力的增大而减小。动剪切模量随循环次数的增加出现先减小后增加再减小的变化,随含水率的升高有较大幅度的降低。动阻尼比随循环次数的增加先增加后减小,随含水率的升高而增大。表明海东地区原状黄土所具有的大孔隙架空结构使其在重复循环荷载作用下易发生振动变形。     

长河坝—黄金坪水库区域地震台网监测能力分析

位于四川省甘孜藏族自治州康定市内大渡河干流的长河坝-黄金坪水电站,为大渡河流域开发中,以长河坝、大岗山、瀑布沟等形成主要梯级格局开发方案中,"三库22级"水电站中的第10和11级电站,其地震监测台网也采用梯级设计方式,长河坝-黄金坪上下游水库地震监测台网统一设计、建设及监测,共享同一地震监测能力。通过对监测设备的技术指标、台基地噪声水平和震级-频度对数关系的分析,结果表明,长河坝-黄金坪水库区域地震固定台站建成后,地震监测能力达到并优于设计的理论监测能力,地震监测震级达到下限为M_L 0.5的设计要求。     

特高压互连电气设备减震性能振动台试验

为研究铅金属减震器对特高压互连电气设备的减震效果,进行由硬管母线连接的特高压避雷器设备和电容式电压互感器设备组成的互连耦合体系的地震模拟振动台试验。通过白噪声扫频、抗震及减震试验,测定互连耦合体系抗震结构及减震结构的自振频率以及关键部位的应变、加速度响应。试验结果表明:抗震结构中避雷器设备的最大应变响应大于互感器设备,互连耦合体系中的避雷器设备属于易损设备;安装减震器后互连设备频率降低幅度较小,减震器基本不会影响电气设备的正常运行;安装减震器后互连耦合体系中的避雷器设备和互感器设备在较大峰值加速度地震波作用下,设备顶端加速度响应和最大应变响应均有较大幅度的降低,避雷器设备和互感器设备的最大应变响应的减震效率分别为75%和50%,减震效果显著,减震器的应用大幅提升了互连耦合体系的抗震能力。     

新型半主动调谐质量阻尼器减震性能的数值模拟

为了改良被动式调谐质量阻尼器(TMD)对建筑结构的减震效果,本文提出了一种新型的可实时调节频率和电涡流阻尼的半主动调谐质量阻尼器(SATMD)。由Hilbert-Huang变换(HHT)识别出结构的瞬时频率,通过基于HHT的控制算法实时调节SATMD的质量进行频率的调谐;通过基于线性二次型高斯(LQG)的控制算法实时调整磁导间距来调节电涡流阻尼系数。为了验证SATMD对建筑结构的减震效果,以一单自由度结构模型为例进行地震响应模拟,同时采用一经优化设计的被动TMD作为对比,并考虑由于主结构的累积损伤等引起自身频率下降而造成被动TMD的去谐效应。以主结构的加速度和位移时程峰值、整体均方根值及TMD的耗能性能作为评价指标,对比了SATMD在主结构发生损伤前后对被动TMD的改良效果。数值模拟结果表明,在主结构发生损伤前后,SATMD均比经优化设计的被动TMD有更好的减震效果及耗能能力。     

Direct finite element method for nonlinear earthquake analysis of concrete dams: Simplification,modeling, and practical application

A direct finite element (FE) method for nonlinear response history analysis of semi-unbounded dam-water-foundation systems has recently been presented. The analysis procedure employs standard viscous-damper absorbing boundaries to model the semi-unbounded foundation and fluid domains and specifies the seismic input as effective earthquake forces—determined from a control motion defined at the foundation surface—at these boundaries. Presented in this paper are several simplifications to this direct FE method that greatly facilitates its implementation in commercial FE software. Also addressed is the modeling of the principal nonlinear mechanisms for concrete dams, calibration of damping in the numerical model to ensure consistency with values measured at actual dams, and practical procedures for implementation of the direct FE method with a commercial FE program.     

A novel bidirectional pendulum tuned mass damper using variable homogeneous friction to achieve amplitude-independent control

Passive tuned mass dampers (TMDs) are widely used in controlling structural vibrations. Although their principle is well established, the search for improved arrangements is still under way. This effort has recently produced an innovative paradigm of bidirectional pendulum TMD (BTMD) that, moving along a specially designed three-dimensional (3D) surface, can simultaneously control two in-plane orthogonal structural modes. In existing versions of BTMDs, energy dissipation is provided either by ordinary horizontal viscous dampers or by an original arrangement of vertical friction dampers. In this paper, a new paradigm is proposed, in which energy dissipation comes from the tangential friction arising along the pendulum surface out of an optimal spatially variable friction coefficient pattern. Within this paradigm, if the friction coefficient is taken proportional to the modulus of the pendulum surface gradient, the dissipation model results nonlinear homogeneous in the small-displacement domain, and the performance of the absorber, herein called the homogeneous tangential friction BTMD (HT-BTMD), results independent from the excitation level. The present work introduces this concept, derives the analytical model of the HT-BTMD, establishes a method for its optimal design, and numerically verifies its seismic effectiveness in comparison with viscously damped devices. The validity and feasibility of the concept are demonstrated through experimental tests on a small-scale lab prototype, which also show the efficacy of a stepwise approximation of the homogeneous friction pattern. The new device proves a competing alternative to existing BTMDs, and homogeneous tangential friction proves a promising new paradigm to provide pendular systems with amplitude-independent structural damping.     

Numerical artifacts associated with Rayleigh and modal damping models of inelastic structures with massless coordinates

The paper presents a detailed reexamination of the effects of three damping models on the inelastic seismic response of structures with massless degrees of freedom. The models considered correspond to (a) Rayleigh damping based on current properties (tangent stiffness), (b) Rayleigh damping based on initial properties, and (c) modal damping. The results suggest that some nonzero damping forces/moments at massless DOFs obtained in multistory frames for the case of Rayleigh damping with tangent stiffness may be numerical artifacts rather than a deficiency of the damping model. The results also indicate that significant artificial numerical oscillations in the velocities of the secondary components of MDOF structures are introduced when modal damping or mass-proportional damping is used.     

关于土层地震反应分析中阻尼矩阵建模方式的讨论

阻尼矩阵的选择对土层时域内地震反应计算的准确性有重要意义。本文构造了质量比例阻尼、刚度比例阻尼、Rayleigh阻尼、Caughey阻尼和Clough阻尼矩阵,研究各阻尼对土层地震反应分析精度的影响。以苏通大桥5号桥塔基础处的工程场地为例,分别以人工合成基岩波,Northridge波、Parkfield波和汶川波为输入,得到不同阻尼模型下土层的地震反应,结果表明Clough阻尼在计算精度和误差稳定性上要优于其他阻尼模型,针对Clough阻尼的不足之处提出了改进方法,算例表明改进后的Clough阻尼能在保证计算精度的前提下减少计算机的存储量和计算量。     

铜川新区黄土与古土壤动剪切模量比和阻尼比的统计研究

利用铜川市地震小区划项目的大量典型土样动三轴试验结果,采用双曲线模型拟合得到了铜川新区黄土和古土壤在8个典型应变下的动剪切模量比和阻尼比值。根据不同沉积年代和不同土层深度2种方法对黄土、古土壤动剪切模量比和阻尼比随剪应变变化情况进行了统计和对比分析。研究发现:同一剪应变下,黄土、古土壤动剪切模量比随沉积年代和土层深度的增加而增加。二者对黄土动剪切模量比的影响较为显著。